This invention relates to a drum mixer asphalt plant used to produce a variety of asphalt compositions. More specifically, this invention relates to a drum mixer in which the region for the introduction of liquid asphalt and mineral fines, as well as recycle asphalt material, is isolated from hot combustion gases used to dry and heat the aggregate material.
Several techniques and numerous equipment arrangements for the preparation of asphaltic cement, also referred by the trade as "hotmix" or "HMA", are known in the prior art. Particularly relevant to the present invention is the production of asphalt compositions in a drum mixer asphalt plant. Typically, water-laden virgin aggregates are heated and dried within a rotating, open-ended drum mixer through radiant, convective and conductive heat transfer from a stream of hot gases produced by a burner flame. The prior art consistently teaches that the burner flame and the aggregate material are located at the same end of the drum mixer such that the hot gas stream and the aggregate material pass through the drum mixer in co-current flow.
As the aggregate material flows through the drum mixer, it is combined with liquid asphalt and mineral binder or "fines" to produce an asphalt composition. Exposing the liquid asphalt to excessive temperatures within the drum mixer or in close proximity with the burner flame causes serious product degradation, in addition to health and safety hazards. As a result, various attempts have been proposed to help minimize combustion of the liquid asphalt necessary in the process.
Paddles or flighting mounted on the interior of the mixer have been used to shield the liquid asphalt from the burner flame by creating a curtain of falling aggregate material disposed between the burner flame and the asphalt. While the flighting reduces the likelihood of combustion of the asphalt, the stream of hot gases emitted by the burner flame may still heat the asphalt to an excessive temperature. In such event, the more volatile components of the asphalt are released and the final product may become unfit for use in paving operations.
Excessive heating of asphalt compositions also results in a substantial air pollution control problem, known as "blue-smoke", caused when hydrocarbon constituents of asphalt are driven off and released into the atmosphere. Significant investments and efforts have been made by the industry in attempting to control blue-smoke emissions.
Improvement is also needed in those drum mixers which recycle asphaltic cement removed from road surfaces. In these mixers, the recycle material is ground to a suitable size and mixed with the virgin aggregate prior to mixing with the asphalt. The presence of asphalt in the recycle material creates essentially the same problems in asphalt production as does the presence of liquid asphalt. The volatile components of the asphalt are released upon exposure to high temperatures and carried in the exhaust gases to the air pollution control equipment, typically a baghouse. Within the baghouse, the blue-smoke or tiny particles of asphalt will condense on the filter bags reducing their efficiency and presenting a serious fire hazard. The useful life of the fabric filter used in the baghouse is also reduced when contaminated with asphalt.
The foregoing disadvantages and problem areas are characteristically compounded with the traditional requirement that the asphalt material with the drum mixer flows in the same direction (i.e., co-current flow) as the hot gases for heating and drying the aggregate. Thus, the asphalt component of recycle material and liquid asphalt itself is, by necessity, in direct contact with the hot gas stream and, in some instances, even the burner flame itself.
The need remains in the asphalt industry for improved drum mixer design and operating techniques to address the problems and drawbacks heretofore experienced. The primary objective of this invention is to meet this need.